CI: Adopt the self-repository reference syntax

[ModeSevenIndustrialSolutions]

Why now

The pre-commit autoupdate pull request (#55) is blocked, and this is what
blocks it. That update lifts zizmor-pre-commit to v1.30.1, whose new
self-repository audit reports every ./ call:

zizmor ... Failed (exit code 12)
help[self-repository]: use GitHub's dedicated self-repository syntax
      uses: ./.github/workflows/...
            ^^^ use '$/...' instead of './...'

What changed

Converted 1 call site in testing.yaml to the self-repository form.

A uses: value beginning $/ resolves to the workflow's own repository at
the commit already running, with no checkout. GitHub added it in July 2026
and now recommends it for sibling workflows.

./ and $/ resolve identically here. For a reusable workflow call GitHub takes the called workflow from the caller's commit either way, so neither depends on checkout state and neither can go stale. What changes is the spelling GitHub documents as correct: $/ is now the recommended form for a workflow in the same repository.

The stronger guarantee people attach to $/ belongs to actions, not workflows. A step's uses: ./path really is workspace-relative and needs a checkout first; the job-level uses: converted here never was. The two spellings look alike and the distinction is easy to get backwards, so it is written down rather than left to inference.

One caveat travels with the form: $/ does not exist on GitHub Enterprise Server. Nothing here is affected, because the reference is internal to a workflow that only ever runs in this repository, but anyone copying the pattern into GHES still needs ./.

actionlint still rejects the form

So .github/actionlint.yaml gains a path-scoped ignore, placed as a new entry for the affected file(s).
The pattern matches only a value beginning $/:

- 'reusable workflow call "\$/.+" at "uses" is not following the format'

Anything else malformed at uses: still fails, so this buys compatibility
with the new syntax rather than switching the check off. The comment carries
the condition for removing it.

Verification

Checked against the zizmor version the autoupdate introduces, before and
after the change:

self-repository findings   before=2   after=0

Both pinned pre-commit hooks pass on the result:

actionlint ... Passed
zizmor ...... Passed

Also run: prek run --all-files (all hooks, no files modified) and
aislop ci --changes (0 errors, 0 warnings).

Once this merges, #55 should go green on a rebase or a re-run.

[Copilot]

🟢 Approval recommended

The syntax, documentation, and narrowly scoped lint exception are consistent and complete.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[zxiiro]

🤖 Auto-approved by agent: reviewed workflow/code change for security and CI/CD impact, found low risk. Switches same-repo reusable workflow calls from ./ to GitHub $/ self-repository form (resolves identically at the running commit); narrow path-scoped actionlint ignores only; no permissions/secrets/trigger changes. CI green.